In these years, flat panel displays (FPDs) have been remarkably developed and have been replacing cathode-ray tube monitors. Liquid crystal displays (LCDs) which are the pioneer of the FPDs, in particular, have made remarkable technological progress enough to be commonly used in our daily lives, and their development is further expected.
However, LCDs still have major disadvantages. A typical example of the disadvantages is that LCDs are not good at displaying moving images. This is partly because the response speed of liquid crystal is low. The response speed of liquid crystal has been typically considered as the speed of switching between black and white. In such circumstances as LCDs have been replacing cathode-ray tube monitors, however, switching between halftone level and halftone level is important. Therefore the response speed of liquid crystal should be considered as the speed of the halftone-halftone switching. Moreover, the response speed is generally lower in the halftone-halftone switching than in the black and white switching, thus causing a problem.
Increasing the response speed is thus indispensable for applying LCDs, instead of cathode-ray tube displays, to televisions. The major issue is accordingly how to increase the response speed of liquid crystal with respect to switching between any tones. One of the means to solve the problem is an Overshoot (OS) driving method. FIG. 12 shows an example of the OS driving method. When the tone level of liquid crystal is switched from A to B, the switching speed of liquid crystal is generally higher if a tone level difference between A and B is larger.
Namely, in rise response where A<B as shown in FIG. 12, the switching speed of liquid crystal can be higher than the normal speed of switching from A to B, by instantly inputting an OS tone level C which is higher than B and then inputting the target tone level B. Further, in decay response where A>B, the switching speed of liquid crystal can be higher than the normal speed of switching from A to B, by instantly inputting an OS tone level C which is lower than B and then inputting the target tone level B.
Practically, the switching speed of liquid crystal is highest in full tone switching (for example, switching from 0 tone to 255th tone). Thus, in the OS driving method, the response speed of liquid crystal with respect to switching between any tones can be theoretically increased to the response speed in the full tone switching. It is thus possible to obtain an LCD capable of achieving sufficiently high response speed with respect to switching between any tones by applying the OS driving method to a liquid crystal display mode where the response speed in the full tone switching is sufficiently high.
For realizing the OS driving method as described above, a major issue to be carefully considered is how to determine an OS parameter (signal level to be applied as the OS tone level C in actual driving). In a circuit for performing the OS driving, a commonly used algorism is essentially such that “compare a tone in an n field (present tone A) to a tone in an (n+1) field (attainment tone B); refer to a Look-up Table (LUT); and determine an OS parameter C.” The LUT is a list which determines C using combination of a value A and a value B, such that “the OS parameter C is 190th tone with respect to 120th tone in an n field and 150th tone in an (n+1) field,” for example. If the LUT cannot be accurately determined, the display cannot achieve proper display because of the following conditions.
Namely, if the OS parameter C is properly set, it is possible to achieve an ideal response characteristic which reaches the attainment tone B in one field period without exceeding the attainment tone B, as shown in FIG. 13. On the other hand, if the OS parameter C is set too large in order to obtain sufficiently high response speed, the response waveform of liquid crystal has a corner, as shown in FIG. 14. In such a case, the liquid crystal is overly responding more than the required level of switching. When the LCD in this condition is actually viewed, the LCD unnaturally glares on the occasion of switching and displaying moving images. Further, if the level of the OS parameter C is set too low in order to prevent the overresponse of the liquid crystal, the liquid crystal cannot achieve a sufficient response characteristic (the response waveform does not reach the attainment tone B in one field period), as shown in FIG. 14. As the level of the OS parameter C becomes lower, the response characteristic becomes more and more similar to the response characteristic without the OS driving as shown in FIG. 15, reducing the advantage of the OS driving.
Therefore the OS parameter C is determined by obtaining a capacity change of the cell that occurs in the switching of liquid crystal and then calculating the OS parameter C using the capacity change of the cell, in a typical conventional technique as shown in Japanese Unexamined Patent Publication No. 352450/1999 (Tokukaihei 11-352450; published on Dec. 24, 1999).
Further, a method to estimate the OS parameter C using response waveform in normal driving has been devised as described below.
1. Response waveform between tones in the normal driving method is measured.
2. The tone level attained after a period corresponding to the application of an OS signal is obtained from the waveform.
3. With this result, an LUT is created by estimating the OS parameter C required for the switching between the tones.
If the OS parameter C is calculated using the capacity change, however, the response does not take into account the influence of the viscosity of liquid crystal at all. This often causes a large difference between the calculation result and the actually required OS parameter C. Namely, the OS parameter should be uniquely determined depending on the thickness and shape of the cell, liquid crystal material, and the like. In practice, however, the OS parameter has a large error of not less than 30% in some portion. Since the user had lower tolerance level with respect to error in the parameter, a certain degree of error in the parameter had been permitted as long as liquid crystal whose response is slow can respond at a speed barely enough to display moving images. However, since the tolerance level with respect to error in the parameter has become extremely higher because high-speed response as well as a finer image quality has been required, the conventional method has become insufficient for obtaining the OS parameter C having a small error.
Further, though the method to estimate the OS parameter C using the response waveform satisfies all required conditions, this method is complicated because it is required to check the waveform by performing the OS driving using the created LUT, in order to correctly determine the OS parameter C. Further, if the waveform needs to be adjusted as a result of the checking, the estimation and measurement for the checking need be repeated many times, thus requiring much labor.